GLAST Just Launched!

By Sean Carroll | June 11, 2008 11:11 am

The Gamma-ray Large Area Space Telescope, a satellite observatory designed to — guess what? — measure gamma rays, just launched on a Delta II rocket from Cape Canaveral. There were a few last-minute radar issues, but things seem to have ultimately gone off without a hitch. There is a launch blog here (naturally), and Phil Plait has been covering the mission in detail; there was a nice article in symmetry, and they also have a live blog.

“Vehicle performance continues to look nominal…” You have to love scientists.

GLAST will be doing a variety of cool things, but there is one goal that stands out as uniquely exciting for physicists: it will be searching for dark-matter annihilations. If the dark matter consists of weakly interacting massive particles, they can come together and annihilate into a cascade of lighter particles. (Image from Sky & Telescope.) Among the particles produced are very high-energy photons: gamma-rays. Those are what GLAST will be looking for, a process known as “indirect dark-matter detection,” in contrast to direct detection where a dark-matter particle bumps into an experiment here on Earth.

Of course, dark matter doesn’t annihilate very often, or it all would have gone away by now. The interactions are very infrequent, so you’re most likely to see the gamma-ray signature in areas of high dark-matter density, such as the center of our galaxy or in clusters of galaxies. (The number of annihilations goes as the density squared, so you get a lot more in crowded regions.) We can imagine a future in which dark matter is no longer considered “dark,” so long as you look in the right part of the spectrum, and we use combinations of techniques to map out the dark matter distribution throughout the universe. Cosmologically speaking, the 21st century is going to be the Dark Ages, but in a good way.

It’s not all that easy, of course — sadly, there are other sources of gamma-rays in the universe other than dark-matter annihilations. It’s going to be a task to know for sure whether some individual source of gamma-rays is produced by DM annihilation or some more prosaic mechanism, such as an active galactic nucleus. Apparently, there are people (“astronomers”) who like to study those sources for their own sake, so it’s not a total loss. One way or the other, GLAST is going to be looking at the universe in an exciting new way.

  • Professor R

    Great news, thanks for that Sean!

    Coincidentally, Tim Sumner of the ZEPLIN III DM experiment gave a talk on their project at Trinity College Dublin on Monday night (I have a summary on my blog).
    I meant to ask his opinion on the prospect of success of DM detection by gamma ray observation but unfortunately I forgot.. Cormac

  • Ellipsis

    Given that HESS and MAGIC, etc., etc., don’t see annihilation (or at least a significant, non-background source yet), I’d like to know what sort of signatures would allow / be best for GLAST to make a DM annihilation discovery? Would any people in this area like to comment?

  • Joe

    Neat. Could you explain a little more why the number of annihilations goes as the density squared? I’m trying to do a back-of-the-envelope thing to see why and I’m not sure where to start.

  • Sean

    Ellipsis, I don’t know enough about the relative capabilities of the different observatories, although GLAST is certainly ahead of the pack. Perhaps someone else will chime in.

    Joe, it’s just because you need two particles to bump into each other. So you have a number of particles to consider, times a number of particles for them to bump into, both of which are proportional to density, so it goes as density squared.

  • Eric

    I know next to nothing about MAGIC and HESS, but one difference between them and GLAST that might make a difference to dark matter searches would be the energy range they’re sensitive to. GLAST’s is about 30MeV-300GeV; I know HESS’s extends quite a bit higher than that, but my 2 minutes of googling hasn’t turned up a number for the low end of their range.

    The nicest signal for WIMP detection (the “smoking gun”, as they say), would be a spectral line at the WIMP mass from annihilation of two WIMPs directly into 2 gammas, or a gamma and a Z boson. In the case of supersymmetric dark matter, where the WIMP is (usually) a Majorana fermion (usually the lightest neutralino), annihilation directly into two photons, or a photon and a Z, would be very rare, so the lines would be very faint. I’m not sure to what extent the same is true of, for example, Kaluza-Klein dark matter, where the DM candidate is usually a boson. In any case, the basic signature, aside from the lines, would be a continuous spectrum with a shape that doesn’t look like what you usually expect from astrophysical sources (power laws and similar, for the most part) with a sharp cutoff at the WIMP mass. It would be especially nice if we found a whole bunch of sources with spectra like that, that all looked the same. That would be a pretty clear indicator that they were being produced by the same process, and if the spectrum looked right you could probably make a strong argument for it being dark matter.

  • Ellipsis

    Eric — I think ground-based Cherenkov telescopes go down to about 50 GeV, so indeed significantly lower than that would preclude ground-based Cherenkov detection. But my understanding is that in the low end of GLAST’s range, satellites like Compton and INTEGRAL have not seen any sort of clear evidence for DM annihilation either (I think EGRET on Compton saw some unidentified things, but I don’t think they really looked like DM). Maybe there’s a window right around 40 GeV(?), but how big is that window, and does it correspond to realistic DM models?

  • Joe

    Okay, that makes sense. Thanks!

  • miller

    If they detect the neutralino annihilations, will they also be able to determine its mass based on the frequency of light? And how do we know they’ll annihilate at all if they’re there? Is it because neutralinos are their own antiparticle?

  • Eric

    ellipsis – It looks like HESS’s range was originally 100 GeV – 10 TeV, though they are now making a selection cut at 160 GeV due to some sort of optical degradation. I think you’re right that pretty much covers most of the interesting parameter space, at least for SUSY. It sounds like HESS and MAGIC have put some limits on annihilation cross sections. I’m not familiar enough with the ACT experiments to know what advantages and disadvantages they have compared to GLAST for detecting dark matter. It could be an issue of sensitivity, spatial resolution, energy resolution, or something else. I’m just not sure off the top of my head.

    miller – Yes, in supersymmetric models neutralinos are their own antiparticles, so if you’ve got a bunch of them in the same place you’re bound to get some annihilations. The mass of the WIMP places a limit on the energy of the annihilation products. The total energy in a system of two annihilating WIMPs is basically just twice the rest mass energy of the particle (kinetic energies are generally negligible in these cases), so the highest energy photon you could get out would be one with an energy equal to the mass of the WIMP. So by looking at the frequency spectrum you would be able to determine the WIMP mass by seeing at what frequency the spectrum cuts off (and the locations of any spectral lines).

    And for what it’s worth, I was being silly in my last post. The rarity of annihilations directly to two photons is nothing to do with whether the particles are bosons or fermions, it’s just because they don’t couple to the EM field.

  • Aaron F.

    I’ve heard that GLAST may also be able to test loop quantum gravity by detecting or putting an upper bound on the dispersion of gamma ray burst photons in vacuum, which at least some LQG models predict. Does anyone know more about this?

  • Sean

    Aaron, the last I heard (and things may conceivably have changed) it wasn’t possible to derive general relativity as a low-energy approximation to LQG, so deriving “predictions” is correspondingly difficult. There are hand-waving arguments that LQG should lead to violations of Lorentz invariance that will be testable by GLAST, but they’re not on a firm footing. Testing Lorentz invariance is intrinsically interesting, regardless of the connection to any particular theory of quantum gravity.

  • Lab Lemming

    “Joe, it’s just because you need two particles to bump into each other. So you have a number of particles to consider, times a number of particles for them to bump into, both of which are proportional to density, so it goes as density squared.”

    But won’t they be traveling faster in high density areas due to gravity, and thus covering more area/unit time?

  • Lawrence B. Crowell

    Sean on Jun 11th, 2008 at 8:34 pm

    There are hand-waving arguments that LQG should lead to violations of Lorentz invariance that will be testable by GLAST,

    I think this may be one place where LQG goes astray. There are some LQG ideas about the speed of light being variable near the Planck length. Yet, the speed of light is just a way of telling us how much space equals how much time. It is a proportion, and variable c amount to loopiness where time depends on space, but where that dependency depends further on space and … . Yuck. The breaking of Lorentz invariance also goes against the grain of symmetry “recovery” at higher energy.

    Lawrence B. Crowell

  • Plato

    Calorimeters measure the collective behavior of particles traveling along approximately the same path, and are thus naturally suited for the measurement of jets-Dorigo Tommaso

    I had been following Glast history as well, and the relationship I thought the “PI institute” held in regards to a “time value measurement position” in regards to Phenomenological order to progressing science. This may be a mistaken notion on my part. I thought Lee Smolin was leading in this area and thought the Glast experiment important to his position.

    Longitudinal and Transverse Information about the Energy Deposition Pattern

    The calorimeter design for GLAST produces flashes of light that are used to determine how much energy is in each gamma-ray. A calorimeter (“calorie-meter”) is a device that measures the energy (heat: calor) of a particle when it is totally absorbed. CsI(Tl) bars, arranged in a segmented manner, give both longitudinal and transverse information about the energy deposition pattern. Once a gamma ray penetrates through the anticoincidence shield, the silicon-strip tracker and lead converter planes, it then passes into the cesium-iodide calorimeters. This causes a scintillation reaction in the cesium-iodide, and the resultant light flash is photoelectrically converted to a voltage. This voltage is then digitized, recorded and relayed to earth by the spacecraft’s onboard computer and telemetry antenna. Cesium-iodide blocks are arranged in two perpendicular directions, to provide additional positional information about the shower.

    I have been interested in “calorimetric designs” when I first heard of it in the Glast design.

    Understanding this use in the LHC and hearing of the “onion skin metaphor” was a strange thought, but seems indeed to explain this ability to measure?

    Calorimeters for High Energy Physics experiments – part 1
    Calorimeters for High-Energy Physics – part 2 April 11, 2008

    Tammaso Dorigo gave a nice blog posting in that regard that might give greater clarity to the values of these measures, as well, helps one view the “window of the universe” in a new ways. A point here was the the gamma ray view of the sun some time ago by JoAnne in this Blog.

    While we have ground based measures this allows us to look back at space, and see it in this different light. It all started with the “Fly’s eye?”:) Pierre Auger history, and John Bachall lead in these directions.

  • Jason Dick

    Don’t forget the other part of the instrument, the GLAST Burst Monitor! In addition to the primary gamma ray detection device, GLAST has a number of photomultiplier tubes pointed in different directions for the express purpose of detecting gamma ray bursts (GRB’s).

    The coolest thing about this device is the extraordinarily wide field of view. The burst monitor can detect bursts basically in any direction that’s not hidden by the Earth, so a little over half the sky. The primary instrument will have maximal sensitivity for “only” around 1/5th of the sky at any given time. It will still detect gamma rays from other directions, but it won’t be nearly as good at pinning down their energies.

  • Plato

    Earlier:A point here was the the gamma ray view of the sun some time ago by JoAnne in this Blog.


    The idea remains the same, but I was not correct in the sense of “gamma ray detection,” but did advance the view to include “neutrinos in observation,” in regards to our view of the universe. Our Sun.

    Hopefully, I am not making it confusing for some.

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About Sean Carroll

Sean Carroll is a Senior Research Associate in the Department of Physics at the California Institute of Technology. His research interests include theoretical aspects of cosmology, field theory, and gravitation. His most recent book is The Particle at the End of the Universe, about the Large Hadron Collider and the search for the Higgs boson. Here are some of his favorite blog posts, home page, and email: carroll [at] .


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